How far back do you have to declare

Normally travel insurance providers will make clear how long ago you need to go back with medical history. It varies from company to company so why not just ask them? 
It also varies across different medical conditions , for example my provider wanted to know if I’d ever had asthma but for viruses/infections they were interested only in the past two years. 

I think SEE has the right answer, each question you're asked should qualify the time frame.

If in doubt it's best to 'phone the insurer and talk through the questions to be sure you're answering them correctly and there will be no problem if something goes wrong whilst you're away on hols.

I hope you can get sorted easilly and go travelling soon.